MySQL single table access method

summary

The so-called access method is that you want to find a pile of data , And then put sql Throw it to mysql, then mysql According to your watch ( Indexes , Data volume , Query cost ), give mysql I think the best way to execute , This is the access method .mysql Give these execution methods their own names , Namely ：const,
ref,ref_or_null,range,index,all,index merge

Equivalent query （ Single field single value ）( Walk a single index )

1.const Method

select * from user where id=? //id It's the primary key ( Cluster index )

select * from user where stu_id=? // Student ID is the unique index ( Secondary indexes )

1. If the query condition is   Primary key or unique index , So the way to access the table is const

2. Generally speaking, it is a method to retrieve a record , be used const

3. If the primary key or unique index is multi column , So many columns must be equal , This is when const

4. If the primary key and unique index retrieve null